The present invention relates to a rubber composition for a cap tread and a pneumatic tire having a cap tread using the same.
The use of spike tires and the loading of chains to tires have been carried out for running on an icy and snowy road surface, but when these are used, a road surface is chipped off by metal pins of the spike tires or the chains on the tires and there is generated a dust problem that materials chipped off from the road surface fly in air. Therefore, studless tires replacing spike tires and chains are proposed for running on an icy and snowy road surface.
Since normal tires have remarkably lower frictional coefficients on an icy and snowy road surface in comparison with the case of general road surface and slip easily, materials and designs of studless tires have been investigated. For example, there are reported development of a rubber composition comprising a diene rubber superior in low temperature property and device of increasing surface edge portions by changing unevenness of a tire surface. However, there has been a problem that studless tires have inferior frictional coefficients on an icy and snowy road surface in comparison with spike tires. Further, it is known that inorganic short fibers having a scratching effect are compounded. However, since the short fibers drop off due to stimulation and abrasion caused by running etc., there has been a problem that a scratching effect is lost.
JP2002-47378A discloses a tread rubber composition for a tire comprising specified amounts of a diene rubber, glass fiber, carbon black, silica and silicone rubber powder, in which abrasion resistance and performance on ice are improved and hardening of a rubber due to aging is suppressed. However, performance on snow (steering stability on snow) is not considered at all and improvements are still required.
JP10-168234A discloses a tread rubber composition especially for a race tire comprising specified amounts of a diene rubber, carbon black, silica and a silicone polymer such as silicone rubber powder or liquid silicone rubber, in which wet grip performance is improved. However, performance on snow is not considered at all and improvements are still required.